Three types of tremor are of clinical significance. The tremor of Parkinson's disease occurs at rest and is sup­pressed by activity. It occurs at a frequency of 3 to 7 per second and is discussed in the section on that disease. Intention tremor occurs in an extremity as it approaches its target or as fine distal movements are attempted. It is frequently due to cerebellar dis­ease, it consists of large irregular oscillations at 2 to 4 per second, and it may be related to cerebellar ataxia. Action or postural tremor occurs at a rate of 4 to 8 per second when the muscles of a limb are contracted, as when the arms are outstretched. It is an exaggeration of the physiologic tremor and is increased by anxiety, thyrotoxicosis, or alcohol withdrawal. Benign essential tremor falls into this category clinically. Essential tremor involves mainly the distal part of the extrem­ity, but it may also involve the wrist and proximal fingers, and may mimic a flapping tremor. It is ordinarily bilateral, but may be asymmetric; and the head, face, mouth, or tongue may be involved. During adulthood or middle years, it may become progressively intense to the point of disability. It may be worse with excitement, fatigue, or tension and, in that case, may respond somewhat to a medical program that includes relaxants. It can occur in many members of the same family, some mem­bers having only an exaggeration of a normal physiologic tremor. Patients with the familial form of tremor may exhibit other dyskinesias as well. Essential or intention tremor may occur with Wil­son's disease or multiple sclerosis and may follow cerebrovascular disease, trauma, or encephalitis. There are no consistent neuro­pathologic findings, but the cerebellar pathways may be involved. Alcohol in small doses may improve the symptoms, as is often discovered by the patient prior to visiting the physician. Medical management may include propranolol in those patients in whom it is not contraindicated, suggesting that central beta-adrenergic re­ceptors may be involved.

Stereotactic thalamotomy or campotomy may result in immedi­ate and dramatic relief of the tremor. The thalamic lesion may be in the posterior edge of the ventrolateral nucleus. If essential tremor is the sole problem, results of stereotactic surgery are reported to be good in 72 to 85 percent of cases, but if the tremor is secondary to other neurological degenerative diseases, such as multiple sclerosis, results are somewhat incon­sistent.l0.40 Patients with mental deterioration or spasticity in addi­tion to the tremor respond poorly, with a higher risk of complications or worsening of their neurological status. Tremor following cerebellar stroke or injury, familial tremor, or essential tremor may respond well to stereotactic thalamotomy. It must be assured, however, that the tremor is severe enough to be disabling and that it is the tremor, not ataxia, that is disabling before one considers the patient for surgery. The target for essential or cerebellar tremor is the same as for parkinsonian tremor. Generally the best result is obtained with a lesion in the V.im nucleus of the thalamus, just anterior to the sensory area for the hand, which can be identified by intraopera­tive stimulation or recording.